In order to ensure the driver's view, electroconductive lines (heating wires) are formed as a defogger on the surface of a glass sheet to be used for a rear window of a car in some cases. The defogger is supplied with an electric current through a feeding metal terminal. This metal terminal is provided on a bus bar connected to the defogger. In some cases, a glass antenna may be used for the rear and side windows of a car. In the case of using the glass antenna, electroconductive lines are formed, on the surface of a glass sheet, in a pattern (an antenna pattern) corresponding to the wavelength to be received. A metal terminal also is provided for the feeding point of this antenna pattern.
Generally, the electroconductive line and bus bar are formed by baking a silver (Ag) paste printed onto the surface of the glass sheet. The Ag paste normally contains Ag particles, a glass frit, and a solvent. A metal terminal is fixed onto the electroconductive coating film formed by baking this Ag paste. Conventionally, a metal terminal is soldered using a tin-lead (Sn—Pb-based) solder alloy. Recently, from the viewpoint of environmental protection, it has been demanded to use a lead-free solder in producing car window glass.
However, when a metal terminal is joined to a glass sheet using a lead-free solder alloy, particularly, a Sn-based lead-free alloy, the following problems occur.
First, the electroconductive coating film may melt and flow into the soldered junction, which may impair the appearance of the electroconductive coating film. The bond strength also degrades together with the degradation in appearance.
Second, it tends to be more difficult to ensure the bond strength of the metal terminal as compared to the case of using the Sn—Pb-based alloy. This tendency becomes conspicuous when using a metal terminal having a plurality of joining planes.
Third, cracks may occur at the surface of the glass sheet in the vicinity of the soldered junction due to a rapid temperature change. Even if the cracks caused in the glass sheet are minute, they should be avoided when consideration is given to the long term strength of the glass sheet. This phenomenon also becomes conspicuous when using a metal terminal having a plurality of joining planes.
In conjunction with the second problem, for example, JP-U-61(1986)-37182 discloses that the bond strength increased with an increase in soldered joining area.